1. Field of the Invention
The present invention relates to an image forming device and an image developing method. More particularly, the present invention relates to an electrophotographic image forming device using magnetic carriers and nonmagnetic carriers and an image developing method using the same.
2. Description of the Related Art
Examples of image development used in an image forming device using electrophotography are copiers, printers, facsimiles, and multi-function devices. These devices include two-component development using toner and magnetic carriers, single-component development using only insulative toner or conductive toner, and hybrid development in which only charged toner is attached onto a developing roller and transferred to an electrostatic latent image. The electrostatic latent image is developed using a two-component developing agent in which nonmagnetic toner and magnetic carriers are mixed.
Due to the use of two-component development, electrification of toner is relatively good, and the image forming device can have a relatively long life span. Additionally, uniform beta images can be formed. However, the two-component development also has disadvantages such as enlargement, complex parts, toner scattering, attachment of carriers to a latent image, and degradation of images due to a reduced durability of carriers.
Due to the use of single component development, the size of a developing device can be minimized, while the quality of dot reproduction may be maintained. However, degradation of the developing and charge rollers performance may affect the durability of the developing device. When the toner is empty, the whole developing device must be replaced, so it can be costly. Furthermore, selective development may occur. Selective development denotes attachment of only a portion of toner which has a predetermined weight and a predetermined amount of charge on a developing roller to an electrostatic latent image. When such selective development continues, residual toner, that is, toner having a weight and charge less than the predetermined values, is not used during development. Therefore, a toner usage rate is reduced.
Due to the use of a hybrid development, dot reproduction is relatively good and the lifespan of the developing device can have be increased. Moreover, fast image formation is possible. Japanese Patent Publication Nos. 6-67546, 7-72733 (U.S. Pat. No. 5,420,375), and 7-92804, the entire disclosures of which are hereby incorporated by reference, disclose hybrid developments each using a magnetic roller and a donor roller. In the hybrid developments, toner is supplied to the donor roller by the magnetic roller. Electrodes are installed between the donor roller and a photosensitive conductor. A bias voltage, in which a direct current (DC) and an alternating current (AC) are mixed, is applied to the space between the electrodes and the donor roller to form a toner cloud around the electrodes to develop an electrostatic latent image on the photosensitive conductor.
In the development system, where the electrodes are installed between the donor roller and the photosensitive conductor, irregular development occurs due to a vibration of electrode wires electrically biased and tensed. Alternatively, stripped traces develop on the donor roller due to the instant attachment of dust to the electrodes. To address this problem, Japanese Patent Publication No. 2000-250294, the entire disclosure of which is hereby incorporated by reference, discloses a development system using a donor roller in which the electrodes are buried. Since the development system needs a brush electrode for supplying a bias voltage having overlapped AC and DC to the buried electrodes, the development system is complicated and expensive. Additionally, when the brush electrode is contaminated or toner is fused to the brush electrode, the contact between the brush electrode and the electrodes of the donor roller is difficult. Furthermore, when consecutive image patterns of high concentration are developed, fine powdered toner and a contaminating material produced from the toner attach to the donor roller, thereby causing a toner film on the donor roller. Thus, a toner layer on the donor roller becomes irregular, and image irregularity, such as generation of spots on an image may relatively easily develop.
In the hybrid development, ghost image development can also occur. Ghost image development comprises a phenomenon in which a previous image remains on a currently developed image. Toner supplied to the donor roller by the magnetic roller is partially developed onto the photosensitive conductor. To perform the next development, the magnetic roller supplies toner to the donor roller to supplement the amount of toner consumed during the previous development. At this time, if the amount of toner supplied to the donor roller by the magnetic roller is insufficient, the consumed toner cannot be effectively supplemented. Then, a toner layer on the donor roller has an uneven thickness, so that ghost image development occurs.
Accordingly, there is a need for an improved electrophotographic image forming device and a developing method which prevents the development of image irregularities such as ghost images generated during consecutive print jobs.